Classical Encryption Techniques

1
Cryptographic Algorithms and Protocols

• Classical Encryption Techniques
• Slides 02

2
Symmetric Encryption

• Terminologies
• Substitution Ciphers
• Shift Cipher
• Substitution Cipher
• Caesar Cipher
• Monoalphabetic Cipher
• Playfair Cipher
• Vigener Cipher
• One Time Pad
• Transposition Ciphers
• Rail Fence cipher
• Row Transposition Cipher
• Product Ciphers
• Rotor Machines Cipher
• Steganography

3
Symmetric Encryption
Insecure Channel
Decryption Algorithm
Secure Channel
Key
4
Asymmetric Encryption
Cryptanalyst
Cipher Text
Plain Text
Decipher
Plain Text
5
Terminologies

• Symmetric Encryption
• Conventional encryption
• Single Key encryption
• Private key encryption
• Asymmetric Encryption
• Public Key encryption
• One key is public and the other is secret

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Terminologies

• Plaintext the original message
• Ciphertext the coded message
• Cipher algorithm for transforming plaintext to
  ciphertext
• Key info used in cipher known only to
  sender/receiver
• Encipher (encrypt) converting plaintext to
  ciphertext
• Decipher (decrypt) recovering ciphertext from
  plaintext
• Cryptography study of encryption
  principles/methods
• Cryptanalysis (code-breaking) the study of
  principles/ methods of deciphering ciphertext
  without knowing key
• Cryptology the field of both cryptography and
  cryptanalysis

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Terminologies

• The Encryption Process
• EK(M) C
• where
• M Plain Text
• E Cipher Function
• K Secret Key
• C Cipher Text
• The Decryption Process
• M DK(C)

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Cryptography

• can characterize by
• type of encryption operations used
• substitution / transposition / product
• number of keys used
• single-key or private / two-key or public
• way in which plaintext is processed
• block / stream

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Brute Force Search

• I will try every possible key!
• The time to find the key depends on the key size

10
Caesar/Shift Cipher

• Example
• K 3
• A ? D, B ? E , C ? F, , X ? A , Y ? B Z ? C
• HELLO ? KHOOR
• Click here to try the Java Applet

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Caesar Cipher Cryptanalysis

• Assume a Hacker caught this Cipher message
• GB OR BE ABG GB OR GUNG VF GUR DHRFGVBA
• JURGURE 'GVF ABOYRE VA GUR ZVAQ GB FHSSRE
• GUR FYVATF NAQ NEEBJF BS BHGENTRBHF SBEGHAR,
• BE GB GNXR NEZF NTNVAFG N FRN BS GEBHOYRF,
• NAQ OL BCCBFVAT RAQ GURZ? GB QVR GB FYRRC
• AB ZBER NAQ OL N FYRRC GB FNL JR RAQ
• GUR URNEG-NPUR NAQ GUR GUBHFNAQ ANGHENY FUBPXF
• GUNG SYRFU VF URVE GB, 'GVF N PBAFHZZNGVBA
• QRIBHGYL GB OR JVFU'Q. GB QVR GB FYRRC
• GB FYRRC CREPUNAPR GB QERNZ NL, GURER'F GUR
  EHO
• SBE VA GUNG FYRRC BS QRNGU JUNG QERNZF ZNL PBZR
• JURA JR UNIR FUHSSYRQ BSS GUVF ZBEGNY PBVY,
• ZHFG TVIR HF CNHFR GURER'F GUR ERFCRPG
• GUNG ZNXRF PNYNZVGL BS FB YBAT YVSR
• What is the key?

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Caesar Cipher Cryptanalysis

• GB OR BE ABG GB OR GUNG VF GUR DHRFGVBA
• JURGURE 'GVF ABOYRE VA GUR ZVAQ GB FHSSRE
• GUR FYVATF NAQ NEEBJF BS BHGENTRBHF SBEGHAR,
• BE GB GNXR NEZF NTNVAFG N FRN BS GEBHOYRF,
• NAQ OL BCCBFVAT RAQ GURZ? GB QVR GB FYRRC
• AB ZBER NAQ OL N FYRRC GB FNL JR RAQ
• GUR URNEG-NPUR NAQ GUR GUBHFNAQ ANGHENY FUBPXF
• GUNG SYRFU VF URVE GB, 'GVF N PBAFHZZNGVBA
• QRIBHGYL GB OR JVFU'Q. GB QVR GB FYRRC
• GB FYRRC CREPUNAPR GB QERNZ NL, GURER'F GUR
  EHO
• SBE VA GUNG FYRRC BS QRNGU JUNG QERNZF ZNL PBZR
• JURA JR UNIR FUHSSYRQ BSS GUVF ZBEGNY PBVY,
• ZHFG TVIR HF CNHFR GURER'F GUR ERFCRPG
• GUNG ZNXRF PNYNZVGL BS FB YBAT YVSR
• N could be I or A ? key is 5 or 13
• Comparing Letter frequency between Cipher and
  English texts

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Caesar Cipher Cryptanalysis
Try the shift 1, 6, 13, or 17 The common shift
is 13
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Monoalphabetic Cipher

• Plain abcdefghijklmnopqrstuvwxyz
• CipherDKVQFIBJWPESCXHTMYAUOLRGZN
• EXAMPLE
• Plaintext ifwewishtoreplaceletters
• CiphertextWIRFRWAJUHYFTSDVFSFUUFYA
• Click here to try the Java Applet

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Playfair Cipher

• a 5X5 matrix of letters based on a keyword
• fill in letters of keyword (sans duplicates)
• fill rest of matrix with other letters
• eg. using the keyword CHARLES
• Click here to try the Java Applet

L R A H C
F D B S E
N M K I G
U T Q P O
Z Y X W V
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Vigenère Cipher

• An improvement over the Playfair
• It is a polyalphabetic substitution cipher
• Effectively multiple Caesar ciphers
• Key is multiple letters long K k1 k2 ... kd
• ith letter specifies ith alphabet to use
• Use each alphabet in turn
• Repeat from start after d letters in message
• Decryption simply works in reverse

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Vigenère Cipher
KEY
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Vigenère Cipher

• write the plaintext out
• write the keyword repeated above it
• use each key letter as a caesar cipher key
• encrypt the corresponding plaintext letter
• eg using keyword deceptive
• key deceptivedeceptivedeceptive
• plaintext wearediscoveredsaveyourself
• ciphertextZICVTWQNGRZGVTWAVZHCQYGLMGJ
• Click here to try the Java Applet

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Vigenère Cipher Autokey Cryptanalysis

• Ideally, we need a key as long as the message
• Vigenère proposed the autokey cipher
• Example
• key deceptivewearediscoveredsav
• plaintext wearediscoveredsaveyourself
• ciphertextZICVTWQNGKZEIIGASXSTSLVVWLA
• knowing keyword can recover the first few letters
• Click here to try the Java Applet

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One-Time Pad

• To build unbreakable Cipher using Vigenère, the
  key must be random.
• Each key is used once
• Example
• key sdvbncfhjkledtghyrtustryuil
• plaintext wearediscoveredsaveyourself
• ciphertextZICVTWQNGKZEIIGASXSTSLVVWLA
• Difficult to secure the key
• Click here to try the Java Applet

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One-Time Pad

• Different decryptions using two different keys
• Example
• KEY pxlmvmsydoftyrvzwctnlebnecvgdupathfzzlmnyih
  
• Plain mr mustard with the candlestick in the
  hall
• CipherANKYODKYUREOFJBYOJXMNEOQPGMZLWZ_AUERFPLUYTS
  
• KEY mvbpnvmykbigeejqviptrdycxlqytiyshydbnqbuoq
• Plain miss scarlet with the knife in the library
• CipherANKYODKYUREOFJBYOJXMNEOQPGMZLWZ_AUERFPLUYTS
  
• Click here to try the Java Applet

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Transposition Ciphers

• Hiding the message by rearranging the letter
  order without altering the actual letters used
• Still can be recognised since ciphertext have the
  same frequency distribution as the original text

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Rail Fence cipher

• write message letters out diagonally over a
  number of rows
• then read off cipher row by row
• eg. write message out as
• m e m a t r h t g p r y
• e t e f e t e o a a t
• giving ciphertext
• MEMATRHTGPRYETEFETEOAAT

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Row Transposition Ciphers

• a more complex scheme
• write letters of message out in rows over a
  specified number of columns
• then reorder the columns according to some key
  before reading off the rows
• M attackpostponeduntiltwoam
• Key 4 3 1 2 5 6 7
• Plaintext a t t a c k p
• o s t p o n e
• d u n t i l t
• w o a m x y z
• C TTNAAPTMTSUOAODWCOIXKNLYPETZ

25
Product Ciphers

• ciphers using substitutions or transpositions are
  not secure because of language characteristics
• hence consider using several ciphers in
  succession to make harder, but
• two substitutions make a more complex
  substitution
• two transpositions make more complex
  transposition
• but a substitution followed by a transposition
  makes a new much harder cipher
• this is bridge from classical to modern ciphers

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Rotor Machines

• before modern ciphers, rotor machines were most
  common product cipher
• were widely used in WW2
• German Enigma, Allied Hagelin, Japanese Purple
• implemented a very complex, varying substitution
  cipher
• used a series of cylinders, each giving one
  substitution, which rotated and changed after
  each letter was encrypted
• with 3 cylinders have 26317576 alphabets

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Rotor Machines
A ? B, B ? I, C ? E ltone cyclegt A
? E, B ? Q, C ? T
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Steganography

• an alternative to encryption
• hides existence of message
• using only a subset of letters/words in a longer
  message marked in some way
• using invisible ink
• hiding in LSB in graphic image or sound file
• has drawbacks
• high overhead to hide relatively few info bits

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Summary

• have considered
• classical cipher techniques and terminology
• monoalphabetic substitution ciphers
• cryptanalysis using letter frequencies
• Playfair ciphers
• polyalphabetic ciphers
• transposition ciphers
• product ciphers and rotor machines
• stenography